Method of making fuel elements



Dec. s, 1959 c. H. IBEAN ErAL METHOD oF MAKING FUEL. ELEMENTS Filed April 30, 1957 Unite METHOD F MAKING FUEL ELEMENTS Charles H. Bean, Naperville, and Robert E. Macherey, Villa Park, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission Application April 30, 1957 Serial No. 656,163

7 Claims. (Cl. 29-473.3)

This invention relates to methods for covering a metal core and more particularly to improvements over. the process for enclosing a metal core disclosed and -claimed `in the copending application of Henry A. Saller, Serial No. 323,348, led December 1, 1952, now Patent No. 2,820,751, issued January 21, 1958.

The above-referenced application introduced the picture-frame technique for roll-bonding a casing material to a metal core placing special emphasis on encasing in zirconium, uranium core fuel elements for neutronic reactors. In the disclosed method the uranium core was tted into a frame formed by cutting an aperture into a ,plate of casing material, placing cover sheets on either side, inserting the assembly into a sealed and evacuated steel-jacket, and heating and rolling the complete assembly. This process was somewhatexpensive, however, in that excess material and labor were expended in fabricating the material of the frame. It was also found that the method had failings in that deleterious substances captured within the frame and cover sheets sometimes Vcontaminatedtheir internal surfaces causing inadequate 'zte'rious to the bonding process.

For information on. neutronic reactors and the theory of operation thereof, reference is made to U.S. Patent No. 2,708,656, filed December 19, 1944. Reference isalso made'to the copending application of Zinn, Serial No. 721,108, iiled January 8, 1947, and the copending application of Wigner, Serial No. 314,595, led October 14,

'1952, now' Patent No. 2,831,806, issued AprilV 22, 1958,

for further information regarding reactors capable of utilizing fuel elements fabricated by the processes disclosed herein.

yOther objects of this invention will become readily -apparent by a reading of this specification, especially with respect to the drawings, in which:

Fig. l is a sectional elevation View of the core and casing assembly prior to rolling;

Fig. 2 is a, section view taken along the line 2-2 of Fig. 1;A j

Fig. 3 is a plan view with a portion cut out showing the assembly of Fig. 1 enclosed in a metal jacket preparatory to rolling;

Fig. 4 is a sectional view taken along the line 47-4 of Fig. 3;

Fig. 5 is a sectional view taken along the line 5-75 yof Fig. 3;

Fig. 6..is an. isometric ,view` showing. the o riemation` of atent 2 one of the parts of the assembly of Fig. 1, with respect to machining operations in the fabrication thereof.

In accordance with the teachings of the invention there is provided a process for enclosing a plate of first metal in a second metal comprising forming a framehaving separate parts around the plate, the parts being so oriented and sized with respect to the plates to provide optimum roll-bonding conditions, sealing the assembly while purging its interior with an inert gas, evacuating the interior, placing the assembly in a jacket of a third material, and heating and rolling to bond the assembly together.

In the embodiment illustrated in the drawings, an assembly or billet 10 comprises a uranium alloy plate or core 12, suitably containing 5%. by weightzirconium, and 11/2% by weight niobium. Uranium or other uranium alloys such as uranium-zirconium, uranium-niobium, or uranium-molybdenum alloys may` also be used.. The core 12 is framed with a pair of side pieces 14 and a pair of end pieces 16 fitting between and spacing apart the side pieces at either end (Fig. 3). Each of the end vpieces 16 has a V-shaped groove 1S (Fig. 1) to receive the tapered end 13 of the core 12. An aperture 20 is provided in one of the end pieces 16 and is. adapted to receive a gas supply or a vacuum pump (not shown). The end, pieces 16 are fabricated from rolled bar stock 21 and are oriented as shown in Fig. 6 so thatthe fabrication planeof rolling will be perpendicular to the plane of the core or plate 12 when assembled therewith. Cover sheets 22 are placed on opposite faces of the framed core 12, the side pieces 14, end pieces 16, and cover sheets 22 measured so that the space 24 formed thereby is slightly largery in each direction-than the enclosed core 12. The space 24 is made larger than the core 12 to allow for the different coefficients of expansion between the core 12 and the casing to prevent buckling when the billet is heated in a later operation. The side pieces 14, end'pieces 16, and cover sheets 22 forming the billet casing are fabricated of zirconium or a zirconium-tin alloy such as Zircaloy-Z" which is an alloy comprising the below listed materials:

Weight by percent Tin 1;2-1.6 Iron (L08-0,17 Chromium 0.06-0'.14 Nickel 0.03-0,07 Nitrogen Maximum 0.009 Zirconium Balance of alloy The tapered ends 13 of the core 12 and the V-shaped grooves 18 in the end pieces 16 affordl a transition zone between the end pieces and thecore to reduce the effect of the slight change in rolling characteristics between the core and end plug, thus providing a tight bond between the core and the end pieces in thesubsequent rolling step. The end pieces 16 are oriented as described abovewith respect to the rolling operation in the previous fabrication of the end pieces to ensure effective bonding between the end pieces 16 and the side pieces14- when the billet is rolled. This preferred orientation takesadvantageof `the rolling anisotropy of hexagonal close. packed crystal metals such that maximum spread of the end pieces will occur in a sideways direction and form a better bond with the side pieces 14. Reference is made to Structure of Materials, Charles S. Barret, McGraw Hill, 1952, regarding the rolling anisotropy of metals.Y

The assembled billet is placed between water cooled plates (not shown) and pressure normal to the billetfaces and normal to the side pieces is applied thereto. While. the billet is under pressure an inert gas, suchf as'argon, is introduced into the space 24 through the aperture 20 to prevent contamination of the internal surfaces ofthe Zirca1Qy-2pies.by air-AA While the hilletisuuslrzpres sure and the purge gas is being introduced, all seams formed by mating Zircaloy-Z components are sealed by fusion preferably using inert gas shielded, tungsten arc weldingtorches. Pressure is then removed from the billet and the space 24 is evacuated of purge gas and air. `If the evacuation is not sucient to remove all of the inert purge gas it may be preferable to purge it with air one or more times so that if any gas remains in the billet it will be air. An inert gas, if trapped in suicient quantities in the billet, will cause blisters to form between the core and the casing whereas air will be dissolved in the zirconium containing casing and, to some extent, in the uranium core when the billet is subsequently preheated for rolling. A pin 25 formed of the same material as the casing is then inserted in aperture 20 and welded therein suitably by a vacuum arc welding process to completely seal the interior of the billet from the atmosphere.

Following the sealing and evacuation, the billet is tted into a four-piece channel-type steel jacket 26, Figs. 3, 4, and 5, suitably formed of SAE-1020 steel. The jacket 26 is formed of a pair of channeled members 28 which mate to form a chamber 30 into which the assembled billet its. Plugs 32 t in either end of the chamber 30 thereby completely enclosing the billet 10 within the steel jacket 26. Each of the components of the steel jacket 26 is chamfered so that their matching surfaces form V-shaped grooves 34 along the exterior of the jacket which are sealed at their roots by heli-arc welding and completely filled by conventional arc welding methods. The purpose of the steel jacket is to restrict atmospheric `contamination of the Zircaloy-Z casing during heating and rolling and to provide restraint of the billet particularly against spread during rolling. p

The jacketed billets are then preheated and rolled to the desired size. They are preferably preheated for two hours to a rolling temperature of between approximately 750 C. to 900 C., 850 C. being a suitable temperature, and rolled to an 80% reduction in thickness in twelve passes, the first passes being light (2.5%) to minimize shifting of components and increased to 17% in the final passes. The preferred rolling cycle consists of two passes followed by an 18-minute reheat with the exception of the first and last passes which are single. After rolling, the fuel plates are preferably heat-treated for dimensional stability while still in their steel jackets. The steel jackets can be removed by shearing or planing and the core edges located by radiography for machining of the plates to the desired overall width and length.

While a single embodiment to the instant invention has been presented, various modifications within the scope of the invention will be apparent to those skilled in the art. The invention is not limited by this embodiment, but only by the claims which follow.

What is claimed is: i

l. A process for enclosing a plate of first metal with a second metal, comprising inserting the plate into an aperture in a frame of the second metal, placing a sheet of the second metal on each of opposite faces of the assembled plate and frame, purging with an inert gas the air from the space within the frame and the sheets through a second aperture provided in the frame while sealing the seams between the frame and the sheets, exhausting said space, purging said space with air, reexhausting said space, sealing the second aperture and applying heat and pressure to bond the sheets, the plate and the frame to one another.

2. The process specied in claim l and further comprising, prior to applying heat and pressure, inserting and sealing the assembly in a steel jacket.

3. A process for enclosing in zirconium a plate of zirconium-enriched uranium alloy comprising forming a 90 included angle taper at each end of said plate forming a frame around the plate with a pair of zirconium side pieces and a pair of end pieces iitting between the side pieces and each having a V-shaped groove to receive one of the plate ends, the end pieces oriented in the frame so that the plane of rolling in the previous fabrication of said end pieces is perpendicular to the plane of said plate, placing zirconium sheets on the opposite faces of the frame and plate, the cavity formed by said frame and said sheet being larger in each direction than the plate to allow for their different coefficients of expansion, applying inward pressure normal to the sheets and normal to the side pieces, purging the air from within the frame by introducing argon through an opening provided in one of the end pieces while welding all eX- posed zirconium to zirconium seams thereby forming a gas-tight billet except for said opening, evacuating the interior of said billet through said opening, purging the interior of the billet with air, re-exhausting the interior of the billet, scaling said opening, sealing the billet in a steel jacket, heating the steel jacketed billet, rolling the entire assembly to reduce the plate and the zirconium frame and sheets and bond them to one another, said end and side pieces of the frame also bonded together, and stripping olf the steel jacket.

4. In a process for encasing a plate of uranium-zirconium alloy with zirconium comprising inserting the plate `in the aperture of a zirconium frame, placing a zirconium sheet on each of opposite faces of the assembled plate and frame, inserting the assembly in a steel jacket, and applying heat and pressure to bond the plates, the frame and the sheets to one another, the improvements of forming an included angle taper on each end of said plate forming the frame of a pair of zirconium side pieces and a pair of zirconium `end pieces fitting between the side pieces, orienting the end pieces so that the direction of rolling in previous fabrication thereof is perpendicular to the plane of the plate, each of said end pieces having a V-shaped groove to receive the plate ends, the space within the frame and the sheets formed slightly larger in each direction than the plate to provide for the different coefficients of expansion ofthe uranium-zirconium alloy and zirconium, and prior to inserting the assembly in the steel jacket, purging the air from the space within the frame and the plate with argon, sealing all zirconium to zirconium seams, while exhausting said space within the frame and plates purging the space within the frame and the plate with air, re-exhausting the space. i

5. In a method for encasing a core of zirconiumuranium alloy `with zirconium comprising inserting the core into an aperture in a frame of zirconium, placing a sheet of zirconium on each of opposite sides of the assembled core and frame, welding the sheets to the frame, sealing the assembly in a steel jacket and heating and rolling the steel jacketed assembly, the improvement comprising purging the space within the sheet and the frame with an inert gas while welding, and replacing the `inert gas with air after welding `but priorto sealing in the steel jacket.

6. A process for encasing a plate of first metal with a second metal comprising forming a taper on each end of said plate, forming a frame of second metal around said plate with apair of side pieces and a pair of end pieces, each of said end pieces fitting between said side pieces and having a V-shaped groove thereinto receive the plate ends, placing a sheet of second metal on cach of opposite faces ofthe assembled plateA and frame, applying inward pressureto the assembly` normal to the Sheets and normal to the sides of the frame, punging with an inert gas the air from the space within the` frame and the sheets through an aperture inthe frame While sealing. the` seams between the frame and` the sheets, evacuating the space within the frame andthesheets, .purgingthe space .between the` frame `and the sheets with air,` re-exhausting the space, sealing the aperture, insert- :ing the assembly in a steel jacket, and applyingheat and pressure to bond the sheets, the plate, and the frame to `one another.

'1. .A `process for enclosing a plate of rst metal with a second metal comprising placing a piece of second metal along each side of the plate, inserting an end piece of second metal at each end of the plate between the side pieces so that the plane of rolling in the previous fabrication of the end pieces is perpendicular to the plane of the plate, placing a sheet of second metal on each face of the plate in contact with the side pieces and end pieces, applying inward pressure to the assembly normal to the sheets and normal to the side of the frame, purging with an inert gas the air from the space within the frame and the sheets through an aperture in the frame while sealing the seams between the frame and the sheets, evacuating the space within the frame and the sheets, purging the space within the frame and the sheets with air, re-exhausting the space, sealing the aperture, inserting the assembly in a steel jacket, and applying heat and pressure to bond the sheets, the plate, and the frame t0 one another.

Atomics, June 1955, pp. 157, 182. Nuclear Fuels, Beckerly, publ. by D. Van Nostrand Co., Inc., Princeton, NJ., pages 286-295. 

